1. Field of the Invention
The present invention relates to an integrated device designed to be attached on a contacting substrate such as a printed circuit board. The present invention further relates to an electronic system including a printed circuit board on which an integrated device is attached.
2. Description of the Related Art
Integrated devices such as Ball Grid Arrays (BGA) or Flip-Chip devices comprise solder balls for contacting and attaching of the integrated device to a printed circuit board. The integrated device is fixed onto the printed circuit board by placing the integrated device on the printed circuit board so that solder balls abut on respective contact pads of the printed circuit board. A heating process is applied thereafter so that the solder balls melt and contacting joints between the integrated device and the abutting contact pads of the printed circuit board are developed.
While operating the integrated device, heat is dissipated which leads to thermal mechanical stress due to differing thermal expansion coefficients. For example, the coefficient of thermal expansion (CTE) of a silicon chip is about 2.5 ppm/° K and the CTE for the printed circuit board is about 17 ppm/° K. The difference of the thermal expansion due to an increased operating temperature results in a shear stress affecting the solder joints between the integrated device and the printed circuit board. As the solder joints of the integrated device are spread over the surface area of the integrated device, the solder joints are exposed to different shear stresses. Usually, the shear stress on the solder joints in an outer region of the contacting area between the integrated device and the printed circuit board is higher than the shear stress of the solder joints in an inner region of the contacting area. Shear stress leads to a deterioration of the solder joints which can lead to a breaking of the electrical connection between the contact pad of the printed circuit board and the respective contact of the integrated device.
In order to test and possibly improve the reliability of an electronic system with integrated devices attached on a printed circuit board, the electronic system is initially tested in a temperature cycle test, which is typically carried out in a temperature range between −40° C. to 125° C. This temperature cycle is usually repeated 500 to 1000 times, so that the solder joints experience repeated change in the shear stress. Particularly for the solder joints in the outer region of the contacting area, the repeated cycles of changing shear stress may lead to breaking and a substantive deterioration of the solder joint.
To overcome the issue of the degradation of the solder joints due to shear stress (thermomechanical stress), flexible contact elements such as springs or elastic bumps can be used to provide a thermomechanically reliable contact between the integrated device and the printed circuit boards. Elastic bumps can be produced by using a silicone bump on which a rerouting is deposited, extending from the tip of the elastic bump to a respective contact pad of the integrated device, for memories typically located in a center row of the die. The rerouting may only be soldered at the bump top, so that only the bump top is connected to the printed circuit board during reflow soldering, maintaining the flexibility of the silicone bump. As a result, the elastic bumps can absorb the shear stress so that no breaking or cutting of the electrical connection occurs in thermomechanical stress. The flexible contact element can provide reliability during thermomechanical stress; however, due to its resilient nature it cannot serve to ensure the mechanical stability of the package. Therefore, an additional fixing such as a lid or heatspreader is required. However, the additional fixing of the integrated device on the printed circuit board is space-consuming and increases the costs of the assembly of the electronic system. Another possibility is to use an underfill for mechanical stabilization. If an underfill is used it inflexibly glues the integrated device to the printed circuit board. The shear stress is transferred into bend stress of the complete printed circuit board, leading to thermomechanical and mechanical sturdiness. However, underfilling is a very expensive process step, leads to reliability problems in humidity testing and inhibits double sided assembly of integrated devices on the printed circuit board.